Saving oil-derived energy sources, tightening emission standards for the exhaust gases of diesel engines, and limiting carbon dioxide emissions, cause the majority of countries to look for ways to reduce the impacts of thermal engines on the environment. Alternative biofuels based on vegetable oils and animal fats have recently become all the more widespread. Biodiesel fuels (or biodiesels) are produced via transesterification of vegetable oils (triglycerides of higher fatty acids), such as rapeseed oil, soybean oil, palm oil, sunflower oil, and others, and animal fats, with methanol, more rarely with ethanol or isopropanol, in the presence of potassium or sodium hydroxide to serve as a catalyst. Biodiesels represent mixtures of methyl fatty acid esters; they are environmentally promising fuels on the international market. Biodiesel is used as a fuel for diesel and automobile engines, combined heat and power blocks, ships and boats, as well as for stationary diesel engines of trackless land vehicles with motor drive. Biodiesel is a nontoxic, naturally degradable type of fuel; it is virtually free of sulfur and carcinogenic benzene and is derived from renewable resources which are not conductive to the accumulation of gases that cause the greenhouse effect (CO, CO2, SO2, fine particulates, and volatile organic compounds) as opposed to oil-derived fuel. The advantages of biodiesel include good lubricating characteristics (which prolong the life of the engine), higher cetane numbers, and facilitation of cleaning injectors, fuel pumps, and fuel supply channels.
One drawback of biodiesel fuel is its limited storage stability. This is on account of high contents of methyl esters of unsaturated fatty acids, which progressively deteriorate over time the energy value of this fuel and lead to precipitation (which is recognized as fuel clouding) as a result of oxidative degradation to short-chain products. The products formed in the course of degradation of unsaturated fatty acid esters, namely, peroxides, aldehydes, and free short-chain fatty acids, lead to sparingly soluble precipitations and cause metal corrosion in the engine and injection system, and shorten the life of the engine and its power.
Oil-derived diesel fuels are used with a wide spectrum of additives that improve the oxidation stability and other properties thereof. Biodiesel additives are yet far fewer, but they considerably extend the capabilities of this type of biofuel. A stabilizer additive is especially important for biodiesels derived from vegetable oils with high unsaturated fatty acid contents.
A method is known for improving the stability of biodiesel fuel, comprising addition to the biodiesel of the main antioxidant in an amount of from 10 to 20 000 ppm (parts per million) and further the addition of a secondary antioxidant. Herein, the main antioxidant represents bisphenol and is dissolved in an organic solvent before being added to the biodiesel (see US 2006/0219979 A1 C09K15/04, publ. Oct. 5, 2006).
A drawback of this method consists of the complexity of biodiesel stabilization, wherein the additives used create an insufficiently long-lasting stabilizing effect when added to the biodiesel.
The use of 3,5-di-tort-butyl-4-hydroxytoluene and/or tocopherol in concentrations of up to 500 ppm is known for stabilizing mixtures of methyl esters of fatty acids having carbon numbers of from 12 to 18 and prepared by transesterification of palm oil with methanol (see EP 0189049 A1 C07C69/24, C07C67/62, C11B5/00, publ. Jan. 7, 1986).
The antioxidant additives used in this method create an insufficiently long-lasting stabilizing effect when added to the biodiesel.
Various antioxidant biodiesel additives were studied in “Effect of Antioxidants on the Oxidation stability of Rapeseed Oil Methyl Esters” by Simkovsky, N. M., and Ecker, A., Analytik, 1999, no. 6, pp. 317-318.
Phenylenediamines (such as Irganox L57 and Irganox L74) and sterically hindered phenols (Hitec 4702, BHT (4-methyl-2,6-di-tert-butyltoluene), Ionol CP, Lowinox, and propyl gallate) in an amount of 300 ppm were shown to cause an extremely low stabilizing effect at 120° C. and a little better effect at 100° C. and 90° C.
A method is known for improving the storage stability of diesel biofuel, comprising addition of a liquid initial solution containing 2,6-di-tert-butylhydroxytoluene (BHT) in an amount of 15 to 60 mass % of based on the initial solution dissolved in diesel biofuel, to the diesel biofuel to be stabilized to reach a 2,6-di-tert-butylhydroxytoluene concentration of 0.005 to 2 mass % based on the entire diesel biofuel solution (see patent RU 2340655 C10L1/183, publ. Dec. 10, 2008).
The antioxidant additive used in this method creates an insufficiently long-lasting stabilizing effect when added to the biodiesel fuel in relatively high dosages.
It follows that additives providing a considerable improvement of the storage stability of biodiesel fuel are hitherto unknown.